In general, a steering wheel for an automobile has a structure as shown in FIGS. 1 and 2, in which FIG. 1 is a side view of the steering wheel secured to a steering shaft, and FIG. 2 is a front view thereof. In FIGS. 1 and 2, reference numeral 1 designates a ring-shaped rim forming a grip for the driver, 2 designates a spoke fixed to the inside of the rim 1, and 3 designates a boss fixed to the spoke 2 at the center of the rim 1. The boss 3 serves as a mounting hub for a steering shaft 4. In the steering wheel configured as mentioned above, it has been usual to form not only the core material of the rim 1, but also the spoke 2 and boss 3 from metal such as a carbon steel or the like, weld the core material of the rim 1 to the spoke and the spoke to the boss 3, and mold the outer layer of the rim 1 with rubber or resin.
Since most of such a steering wheel is formed from metal and weighs at least 2 Kg, vibrations are caused in the steering wheel, in particular, when the automobile is running at high speeds. Thus, drawbacks are involved in that steering stability is degraded and discomfort is caused to the driver.
Accordingly, to reduce the weight of the steering wheel without impairing its rigidity and to improve its vibration characteristics, commonly assigned Japanese Kokai publication No. 60-222236 published on Nov. 6, 1985 describes a method for manufacturing a steering wheel which is lightweight and which is formed from fiber reinforced plastic. FIGS. 3 to 8 show this proposed manufacturing method for a steering wheel, in which FIG. 3 is a front view of the steering wheel, FIG. 4 is a sectional view taken along the line IV--IV in FIG. 3, FIG. 5 is a sectional view taken along line V--V in FIG. 3, FIG. 6 is a perspective view of a coil-shaped lightweight core material for producing the rim continuously, FIG. 7 is a partially enlarged view showing a connection at abutting ends of a ring-shaped portion of a predetermined length cut from the coil-shaped core material of FIG. 6, and FIG. 8 is a sectional view taken along line VIII--VIII in FIG. 3.
In the FIGURES, reference numeral 1 designates a rim having a diameter of about 2 cm, manufactured in the following manner. A prepreg 5 cured lightly after impregnation with unidirectional fibers consisting of a sole or composite body of carbon fibers, glass fibers, or aramid fiber is wound around a coil-shaped lightweight core material 9 of foamed urethane foam, and the coil-shaped lightweight core material 9 having the prepreg 5 wound thereon is cut in a predetermined length of a ring shape at a connecting portion with a spoke 2. Both ends of the cut-out portion from the remainder are butted, and an outer molded layer 6 is formed surrounding the prepreg 5 by molding a rubber or resin. The diameter of the rim 1 is reduced at the two connecting portions with the spoke 2. Reference numeral 7 designates a slip-preventing portion having an undulate shape formed on the outer molded layer 6. The spoke 2 is made of metal, and is connected to the rim 1 such that one end of the spoke 2 is in contact with the prepreg 5 wound around the coil-shaped lightweight core material 9 at the diameter reduced connecting portion of the rim 1, and in contact with a joining piece 42 and a U-shaped thin metal sheet 41, and the one end of the spoke 2 is wrapped in a prepreg 5 or a cloth prepreg (not shown) and cured by heating to achieve the connection.
The steering wheel configured as described above is manufactured in a process in which the prepreg 5 is first wound around the coil-shaped lightweight core material 9 at a constant positive angle between 10.degree. and 60.degree. by a winding machine, and then is further wound at a constant negative angle between -10.degree. and -60.degree. so that it intersects the previously wound prepreg turns to form a coil-shaped laminated body. Such body is cut to a length required for the rim 1, and the cut ends of the rim portion are joined to each other to form a joining portion 30 as shown in FIG. 7. Following this, as shown in FIG. 8, it is connected to the spoke 2, wrapped in a prepreg 5, cured by heating, and then, it is placed in a metal mold for molding and the outer molded layer 6 of rubber or resin is formed.
Since the manufacturing method described above includes the process of winding the prepreg around the coil-shaped lightweight core material 9, an expensive winding machine is required, which performs complicated motions to move along a coil-shaped path while rotating about the coil-shaped lightweight core material 9, and thus the costs are increased.
Furthermore, in order that the winding machine moves along the coil-shaped path while rotating about the core material 9, the pitch of the core material must be in a range from about 30 to 40 cm per revolution, and thus, the winding machine must be large in size. Further, after the winding, curing by heating, and laminating, both ends of the laminated core material are closed off by strings or the like, and the core material is cut to into the required lengths for the rim 1. However, in this case, at the time of forming each cut length into a ring-shape, since it is pressed until it becomes flattened, when the pitch or fiber winding angle is large, the prepreg 5 will tend to become loosened and delaminated, and torsion will be caused in the prepreg 5 resulting in an irregular alignment of the fibers. Moreover, since the fibers impregnated with resin are exposed to the outside at the surface of the outer layer as a result of the process for curing by heating, and laminating the prepreg 5, when the driver grips the steering wheel, hurt and discomfort will be caused due to the surface projections of the fibers. To eliminate such discomfort, and also to form the undulation which serves as the slip preventing portion 7, the process for forming the outer molded layer 6 is needed additionally, and this gives rise to a problem in that the cost of equipment and the number of processes are increased.